Aged 18, Locke, the youngest son of a colliery manager, was apprenticed to the railway engineer George Stephenson. He formed a lifelong friendship with Stephenson's son Robert, whom he later called 'the friend of my youth, the companion of my ripening years, and a competitor in the race of life'. With Brunel, Locke and Robert were to dominate the engineering world. In 1829 they collaborated in a study of the relative merits of standing engines (which lie at fixed intervals along the track and pull trains via cables) and locomotives, occasioning a quarrel with the elder Stephenson over the credit for authorship. As a result Stephenson did not appoint Locke as chief engineer for his Liverpool to Birmingham line; however he was overruled. After Locke had completed several important lines, they locked horns again over the 'West Coast' route from Lancaster up into Scotland. Characteristically Locke favoured a direct and economical route, although this would be challenging to build; Stephenson wanted to take the line through flatter country at the cost of increasing its length, and poured scorn on Locke's scheme. Eventually a royal commission chose Locke's proposal. This line was to be his greatest work.

In 1834 Locke married Phoebe McCreery, with whom he adopted a child. He was elected to the Royal Society in 1838. In 1847, having purchased a manor in a 'rotten borough', he entered parliament, where he was outspoken on subjects within his experience. Meanwhile he was busy creating railway lines in Britain and on the Continent. He died during a shooting holiday, seemingly from appendicitis.